A conventional solid state interrupter, as exemplified in FIG. 1, includes a suitable solid state switch 10 which interrupts a current path between a source 11 and a load 12, under the control of make and break signals (at leads 13) applied to an electronic logic controller 14. The make and break signals may be applied manually or from an external control source. A current sensor 16 supplies one or more signals to the electronic controller 14 which processes the information and causes the switch 10 to open, should an overload or short circuit condition be detected. A transient voltage suppressor 18 is connected across the switch to absorb incoming line transients or energy stored in the load and/or line inductances (represented by coil 15) when the switch is opened.
In a conventional solid state interruption system, when the switch interrupts a short circuit current, the current in the current path rises rapidly and causes the current to transfer to the transient absorber which absorbs the energy stored in the line and/or load. For example, using the structure of FIG. 1 having a 22,000 AIC, 240 volt, 3-phase line, the current will attain a level of 280 amps in about 23 microseconds. At this point, the switch must open and interrupt the current. The current then transfers to the transient absorber which absorbs the energy stored in the line and/or load. The switch must be very fast acting or else the current will rise to a very high level and the switch will fail. Unfortunately, because of its necessary fast response time, the switch is highly susceptible to nuisance tripping in the presence of noise and other types of adverse line conditions.
The large overload requirements that a circuit breaker must tolerate are also a significant problem for a solid state interrupter. While most conventional circuit breakers tolerate long-term overloads such as motors without tripping, solid state devices have minimal thermal mass and, therefore, cannot handle large overloads such as those presented with motors. One popular conventional circuit breaker, for example, can tolerate an overload of 35 times its rated level for 0.15 seconds. If overloads of this magnitude had to be carried by a solid state switch, the switch would have to be significantly over-designed.
Another problem associated with known solid state circuit breakers concerns the amount of overload current "let-through" the breaker. This "let-through" current is primarily due to the design of the solid state switch, rather than to the current demands of the load. It is, therefore, important to design the solid state interruption systems to handle anticipated worst case load conditions rather than just duplicate the steady-state performance specification of the breaker.
Different overloads that may be anticipated by solid state interruption systems may be subdivided into one of two types. The first type is a long-term overload, which is imposed by equipment such as motors. The second type, a short-term overload, is defined by a rapidly rising overload imposed by devices such as transformers, capacitors, incandescent bulbs, etc., when they are initially energized. While the former type of overload must be carried in order for a motor to start, the short-term rapidly rising overloads are undesirable transients because they are detrimental to the load itself.
Moreover, it is virtually impossible to distinguish between a rapidly rising current overload caused by a true short circuit and that caused by the inrush of current due to, say, connecting an uncharged capacitor across the line. If the switch is to interrupt the current in say 23 microseconds, at 280 amps, it may be breaking a short circuit appropriately, or it may be stopping a capacitor from being fully charged erroneously. In any case, prior art switches are designed to stop the current at the 280 amp level, because they cannot distinguish between the two overload types.
Accordingly, there is need for a solid state breaker arrangement and technique that overcomes the above deficiencies known to the prior art.